The controlled deposition of a film on a substrate is a critically important process in the electronics and many other industries. One of the parameters which must be tightly controlled during such deposition is the film temperature. For example, this is particularly important for fabricating semiconductor multi-layer structures from silicon (Si) or gallium arsenide (GaAs). It is also critical to obtaining a high-T.sub.c (high critical temperature) superconductor film of optimal quality on a substrate. To date, the lack of an appropriate temperature measurement and control system is one of the main obstacles in high-T.sub.c superconductor film technology.
The recent discovery of superconductivity above 90.degree. K. in the compound YBa.sub.2 Cu.sub.3 O.sub.7-.delta. created the potential for new applications in microwave devices and hybrid semiconductor-superconductor devices. For the first time, superconductivity occurred at temperatures where the properties of semiconductors can be optimized. These promising applications require the deposition of high-quality superconducting thin films on semiconductors.
A high quality superconducting film must exhibit a high critical current density J.sub.c greater than 10.sup.5 Acm.sup.-2 at the operating temperature. A narrow transition to the superconducting state, of the order of a few degrees Kelvin, and a high temperature of zero electrical resistance are also required. The film quality strongly depends on the substrate surface temperature during deposition.
Films deposited onto low temperature substrates require an annealing process to become superconducting. This annealing process, at about 900.degree. C., raises several problems, including chemical reaction and diffusion between the film-substrate composite, which degrade the superconducting properties, and differences in thermal expansion coefficients, which may cause microcracks.
In-situ growth without an annealing process is the preferred method of applying superconducting films. In this procedure, films are deposited on a heated substrate and cooled in an oxygen environment. Laser ablation and sputtering deposition processes have yielded the best films of high-T.sub.c superconductors (D. W. Murphy et al., 1988, "Processing Techniques for the 93K Superconductor Ba.sub.2 YCu.sub.3 O.sub.7, Science, Vol. 241, pp. 922-930). A critical parameter in the in-situ growth process is the temperature of the substrate at the film-substrate interface (H. C. Li et al., 1988, "In-Situ Preparation of a Y--Ba--Cu--O Superconducting Thin Film by Magnetron Sputtering," Appl. Phys. Lett., Vol. 52, pp. 1098-1100). It influences film orientation, stoichiometry and crystallinity. Since the temperature drop across the deposited film is small, the substrate temperature at the film interface is approximately equal to the film temperature. Previous work has shown that this temperature must be held at a precise value to ensure good film quality (R. L. Sandstrom et al., 1988, "Reliable Single-Target Sputtering Process for High-Temperature Superconducting Films and Devices," Appl. Phys. Lett., Vol. 53, pp. 444-446). As it is very difficult to measure directly the substrate temperature during deposition, the temperature of the substrate holder is usually reported. This temperature, however, can be up to 150.degree. K. higher than the substrate temperature (A. Inam et al., 1988, "As-Deposited High T.sub.c And J.sub.c Superconducting Thin Films Made at Low Temperatures," Appl. Phys. Lett., Vol. 53, pp. 908-910). A solution to this problem is required before high-T.sub.c superconductor devices can be commercialized.
This same problem of accurately measuring the temperature of a film growing on a substrate has long been a problem in the semiconductor industry. It has undergone intensive study by a number of research institutes and chip manufacturers, but without producing a practicable solution.
Conventional temperature control systems use thermocouples attached to the substrate holder, because they cannot be attached directly to the substrate. As previously indicated, the temperature of the substrate holder may differ significantly from that of the substrate and since the temperature difference is unknown, this is not an adequate solution to the problem.
Pyrometric non-contact temperature measurement of high-temperature sources has been known for sixty years. Such measurements make use of the Planck radiance formula: EQU N(.lambda.)=.epsilon.(.lambda.)C.sub.1 .lambda..sup.-5 [exp(C.sub.2 /.lambda.T)-1].sup.-1 ( 1)
where: N is the spectral radiance, .epsilon.(.lambda.) is the emissivity of the material at wavelength .lambda., T is the thermodynamic temperature, and C.sub.1 and C.sub.2 are Planck radiation constants. Strictly speaking, many pyrometry techniques use the Wien approximation to the Planck radiance formula EQU N(.lambda.)=.epsilon.(.lambda.)C.sub.1 .lambda..sup.-5 exp(-C.sub.2 /.lambda.T) (2)
which is valid for small wavelengths. By measuring the spectral radiance, N, at a wavelength .lambda., and by supplying the appropriate values for .epsilon., C.sub.1, and C.sub.2, an estimate of the temperature of the source can be calculated.
Historically there have been two distinct techniques for such calculations. The older technique, ratio pyrometry, involves measurement of the radiance at a number of different wavelengths in an attempt to eliminate the emissivity term by making ratios of the measured radiances. In the newer method, multiwavelength pyrometry, radiance emission measurements are also taken at several wavelengths. These data are then processed by a variety of techniques, the most accurate of which are the least-squares-based multiwavelength techniques which involve fitting the radiance data to an assumed emissivity functional form.
Ratio techniques have not, in general, provided adequately accurate temperature estimates for broad industrial usage. The often large inaccuracies of the ratio techniques have been attributed to the fact that they require unrealistic assumptions to be made about the nature of the emissivity, .epsilon., in the Planck formula. Ratio techniques assume that both (in the case of two-color) or certain (in the cases of three-color or four-color) of the emissivities at the measured wavelengths be equal. Multiwavelength, particularly least-squares-based, techniques have been somewhat more successful, largely because they more reasonably assume a wavelength-dependent emissivity function, rather than that all or some of the emissivities are equal. In fact, with certain materials, these techniques have proven to be accurate to within one per cent. With other materials, however, results have been unsatisfactory. It has been assumed by previous investigators that the unsatisfactory results have been due to two sources: first, an incorrect form or lack of sufficient complexity of the assumed emissivity model function; and second, so-called "correlation effects" due to the inability of curve-fitting routines to distinguish in certain circumstances between changes in emissivity and changes in temperature.
Pyrometry faces a particular obstacle when it is applied to substrate temperature measurement in film growth processes. Due to absorption and interference within the film, the substrate emissivity changes with increasing film thickness. Because the spectral emissivity of the film-substrate composite depends on the ratio of the wavelength to the film thickness, the emissivity change with film growth depends on the wavelength.
Attempts have also been made to determine the substrate temperature by analyzing the heat transfer between the substrate and other parts of the deposition chamber. Nevis and Tasone calculated the silicon wafer temperature in sputtering and sputter-etching systems based on thermal radiation and heat transfer (B. E. Nevis et al., 1974, "Low-Voltage Triode Sputtering and Backsputtering with Confined Plasma: Part IV. Heat Transfer Characteristics," J. Vac. Sci. Technol., Vol. 11, pp. 1177-1185). Their results showed that the wafer temperature depends significantly on the surface emissivity. They did not analyze the change of emissivity during film growth. Other researchers investigated substrate heating rates by sputtering, but they did not extend their study to predict the film temperature (J. A. Thornton et al., 1984, "Substrate Heating Rates for Planar and Cylindrical-Post Magnetron Sputtering Sources," Thin Solid Films, Vol. 19, pp. 87-95).
It is an object of this invention to provide a method and apparatus for accurately measuring the temperature of a film as it changes in thickness, which account for the change in emissivity with changing film thickness.